This project investigates how rhesus monkeys and other nonhuman primate species born and raised under different laboratory conditions adapt to placement into environments that contain specific physical and social features of the monkey's natural habitat. Adaptation is assessed by examining behavioral repertoires and by monitoring a variety of physiological systems in these subjects, yielding broad-based indices of relative physical and psychological well-being. The responses of subjects to experimental manipulation of selected features of their respective environments are also assessed in similar fashion. Whenever possible, field data are collected for appropriate comparisons. An additional focus is on investigating the cognitive, behavioral, and social processes involved in adaptation to new settings or circumstances. Capuchin monkeys are employed in many of these studies because they are unique among monkeys species in their propensity to manufacture and use tools to modify their physical environment. Research carried out during FY00 (a) documented developmental changes in endocrine, immunological, and health status indicators among juvenile rhesus monkeys in a wild-living population on Cayo Santiago island (PR), demonstrated long-term stability of individual differences on measures of viral activity and adrenocortical responsiveness, and related such differences to differences in maternal dominance ranking; (b) characterized changes in the relationship with mother and in peer interaction patterns following the birth of a younger sibling for yearling rhesus monkeys living in the LCE's 5-acre enclosure at the NIHAC, and compared such changes with those observed in yearlings in the Cayo Santiago wild rhesus monkey population; (c) characterized individual differences in weaning strategies among rhesus monkey mothers living in the 5-acre field enclosure; (d) documented seasonal changes in hair growth patterns in the same rhesus monkey population that were associated with both photoperiod variation and hormonal status; (e) demonstrated that various environmental enrichment deveices (puzzle feeders, grooming boards, and toys) produced only transient reductions in stereotypic behavior and no significant changes in self-directed biting among individually housed rhesus monkey adults; (f) characterized the development of social grooming patterns and conflict resolution over the first year of life in monkeys reared in the absence of adults; (g) discovered a female analog to the male baculum in capuchin monkeys and documented age-related changes in that structure; (h) characterized changes in steroid hormone secretion associated with aging in adult female capuchin monkeys; and (i) identified significant relationships among body temperature, steroid hormone status, and specific behavior patterns associated with thermoregulation in female capuchin monkeys.